Devices called surface light-emitting devices or sheet-type luminous bodies can emit light uniformly from the entire area of the light-emitting surface. Such devices are disclosed in the Published Japanese translation of PCT International Patent Application No. 10-506725, Japanese Patent Applications Laid-open No. 5-224020, No. 7-5326, No. 7-182908, No. 6-265892, No. 1-45003, No. 1-45002, No. 11-154406, No. 4-101639, and the like.
The sheet-type luminous bodies disclosed in these publications generally have the following structure. Specifically, these sheet-type luminous bodies include (1) a body which outlines a light guiding space and has at least one light-emitting surface, (2) a light source which is disposed outside the light guiding space of the body and supplies light to the light guiding space, and (3) a light unifying means which enables uniform brightness to be obtained over almost the entire area of the light-emitting surface when the light supplied to the light guiding space is emitted toward the outside of the body through the light-emitting surface. The body is in the shape of a box of which the sides other than the light-emitting surface are opaque, for example.
The light guiding space outlined by the body is almost in the shape of a rectangular parallelepiped having a specific length, width, and height. Generally, at least one of the sides having the largest area (for example, two sides having two sides parallel in the longitudinal direction and two sides parallel in the widthwise direction of the light guiding space) is an emission plane. The body includes a light-transmitting plate disposed to cover the emission plane. The surface of the light-transmitting plate is the light-emitting surface.
The light source is generally a line light source such as a fluorescent tube or a cold cathode tube. The light source emits light uniformly over the entire area of the circumferential surface and in the longitudinal direction. In the case of using the line light source, the line light source is generally disposed so that the line light source is parallel to both an incident plane and the light-emitting surface in order to cause the brightness of the light-emitting surface to become uniform. The line light source is generally disposed near at least one of four sides which intersect the emission plane (light-emitting surface) at right angles. In this case, the sides other than the incident plane and the emission plane are generally covered with side members formed of an opaque plate or sheet.
The brightness of the light-emitting surface is highest in the area close to the light source and decreases as the distance from the light source increases, whereby the light is emitted nonuniformly. Therefore, it is necessary to use the light unifying means so that uniform brightness is obtained over the entire area of the light-emitting surface. The light unifying means is a prism sheet or a white translucent diffuse transmission film as disclosed in the above publications. Combined use of this sheet and film is particularly effective. The light unifying means is disposed to substantially cover the entire area of the emission plane uniformly. For example, a diffuse transmission film is disposed on the surface of the light-transmitting plate and a prism sheet is disposed on the back face of the light-transmitting plate so that light supplied from the light source is transmitted through the prism sheet and the diffuse transmission film and emitted toward the outside. The prism sheet is generally formed of a transparent resin and has a prism surface on which a plurality of minute parallel prisms is formed.
As the line light source, a line light source including a side emitting type light fiber or a hollow light tube as an optical transmitter is also useful. The optical transmitter generally has a cylindrical circumferential surface (side). Light introduced into the optical transmitter from one tip of the optical transmitter in the longitudinal direction gradually leaks out from the circumferential surface as the light is transmitted toward the other tip in the longitudinal direction, whereby light is emitted brightly from the entire area of the circumferential surface. Japanese Patent Application Laid-open No. 11-142652 discloses an example in which the side emitting type light fiber is used as the line light source of the surface light-emitting device, for example. Japanese Patent No. 2628858 and Japanese Patent Applications Laid-open No. 10-82902 and No. 2000-137105 disclose line light sources using an optical transmitter consisting of a light tube formed by cylindrically rolling up a prism sheet. As disclosed in these publications, the light tube is generally formed so that the prism surface of the prism sheet faces the outside and the inner surface of the cylinder is formed by the flat surface of the prism sheet.
Applications of the surface light-emitting devices capable of emitting light uniformly by using the light source disposed outside of the body (edge lighting light source) and the light unifying means are limited to small light-emitting devices (backlight of a liquid crystal display for personal computers, for example). Specifically, since the area of the light-emitting surface is comparatively small (generally 80×80 cm or less), these surface light-emitting devices are unsuitable for use as comparatively large displays or planar illumination devices disposed on the indoor plane of the building and the like by forming a light-transmitting plate having a large area by horizontally arranging a plurality of light-transmitting plates, and emitting light from the surface of the large light-transmitting plate. The reasons therefor are as follows.
There is a tendency for the brightness of the light-emitting surface to be highest in the area close to the light source and to decrease as the distance from the light source increases. In the case where the area of the light-emitting surface is increased, in particular, in the case where the length of the light-emitting surface of the surface light-emitting device is greater than the width (the length of the light guiding space is greater than the width), the light unifying means disposed uniformly along the light-emitting surface is not effective for increasing the uniformity of brightness. For example, in the case where the light source is disposed near one of the edges of the light guiding space in the longitudinal direction, the distance between the incident plane and the side opposite to the incident plane in the longitudinal direction is increased. As a result, the difference in brightness tends to be increased between the area near the incident plane and the area near the side opposite to the incident plane. In order to eliminate unevenness of the brightness, diffusiveness of the diffuse transmission film must be increased as far as possible. However, this results in a decrease in light transmittance of the diffuse transmission film, whereby the brightness of the light-emitting surface is decreased. Increasing the height of the light guiding space (increasing the depth) is effective for uniformity of brightness. However, this results in an increase in the space occupied by the entire surface light-emitting device. In particular, in the case of emitting light from the indoor plane by incorporating the surface light-emitting device in a partition (floor, wall, or ceiling) of the building, since the depth (thickness) of the partition is limited, the height of the surface light-emitting device must be decreased as much as possible from the viewpoint of the design of the building.
Moreover, since the power consumption is increased by increasing the luminous brightness of the light source or the number of light sources in order to prevent a decrease in brightness, these measures are unfavorable from the viewpoint of energy saving and the like. In the case where the light source is disposed near one of the edges of the light guiding space in the widthwise direction, the distance between the incident plane and the side opposite to the incident plane becomes comparatively small, whereby the difference in brightness is decreased between the area near the incident plane and the area near the side opposite to the incident plane. In this case, a comparatively long line light source having the same length as the light guiding space is necessary. Since a large amount of electric power is needed to emit light brightly from such a light source, the power consumption tends to increase.
Accordingly, the present invention provides a surface light-emitting device capable of preventing an increase in power consumption even if the length of the light-emitting surface of the surface light-emitting device is greater than the width, and capable of emitting light uniformly without decreasing the brightness of the light-emitting surface nor increasing the height of the light guiding space.